EP1021468B1 - Method for reducing residual monomers in liquid systems by adding an oxidation-reduction initiator system - Google Patents

Description

The invention relates to a method for reducing the residual monomer content in a liquid polymer system by post-polymerization with the addition of a redox initiator system.

In the usual radical initiated polymerization of olefinic unsaturated monomers or monomer mixtures Polymerization reaction usually only up to a monomer conversion from 90 to 99% by weight, regardless of whether the polymerization in Solution, carried out in bulk, in suspension or dispersion becomes. The reasons for the incomplete reaction are known to the person skilled in the art of the monomers known (Trommsdorf-Norrish effect, reduction the rate of diffusion, transfer and branching reactions Etc.). The resulting unconverted system Monomers ("residual monomers") are of different types Unwanted reasons (reduced sales, polymer contamination, smell, Toxicity and / or flammability of the monomers etc.). It has therefore not lacked attempts after the main polymerization to remove or lower the remaining amount of residual monomers. This process step is intended as post-polymerization below be understood.

Among other things, known by treatment with water vapor residual monomers to remove in a kind of steam distillation (see e.g. EP-A 327006, EP-A 650977 or US-A 4 529 753). The treatment is complex, however, it is only for aqueous systems feasible, and their success depends on the volatility of those to be removed Monomers dependent. It is common to use the approach Main polymerization add polymerization initiators again and aftertreatment at a suitable polymerization temperature, a so-called post-polymerization, for reduction to carry out the residual amount of monomers. For that is the addition of Redox initiator systems have been described many times (see e.g. US-A 4,289,823, EP-A 9258, EP-A 241127, EP-A 357287, EP-A 417 960, EP-A 455 379, EP-A 474415, EP-A 492847, EP-A 522791, EP-A 623659). A double aftertreatment is often used in EP-A 9258 even a three-time post-treatment is recommended to remove residual monomers to complete polymerization. Repeated post-treatment is undesirable, around the polymerization reactor as quickly as possible to be able to use for his main task. Even the multitude of Approaches suggested in the prior art show that none there is a satisfactory solution. Such as. in EP-A 279892 is specified, the success of the corresponding aftertreatment depends also on the size of the reactor used. For example, the temperature control and the homogenization behavior of large production reactors significantly different from that of laboratory reactors, and a simple transfer of the experiences described to reduce residual monomers on large production reactors is usually not possible.

The present invention has for its object a large-scale workable procedure for a successful Residual monomer reduction through post-polymerization in a liquid system in a production reactor, including a reactor with more should be understood as 20 and preferably more than 100 liters, to provide.

It has now become an effective method for reducing the Residual monomer content of a liquid solution, mixture, melt, Suspension or dispersion of one by radical polymerization produced polymer by post-polymerization with addition a redox initiator system at a suitable reaction temperature found, which is characterized in that at least one of the initiators for the polymerization of the residual monomers required components of a redox initiator to the reaction mixture in a production reactor with a specific, as short as possible Mixing time of the liquid system in the production reactor, gradually, in portions or continuously for a period of time (Dosing time), which is about 10 to 250 and in particular 20 to 100 times the mixing time of the liquid system in the production reactor is.

An important procedural parameter in the process for the targeted reduction of the residual monomers in the liquid polymer system the mixing time  of the liquid system plays a role in the production reactor used, including the one required Time is understood to be through mixing and especially stirring to achieve a certain degree of homogenization. For determining the mixing time are usually the Schlieren method and the chemical decolorization method used. In the chemical Decolorization method is the liquid with a reactant offset and stained with an indicator. at the start of mixing or stirring, the second reactant is then added and the time when the color disappeared was measured. The the degree of homogenization present in the color change depends on Excess of the added reaction component.

der Rührerdrehzahl n und des Rührerdurchmessers d kann unter Beachtung der Randbedingungen die Reynoldszahl des System berechnet und aus dieser die Mischzeit  bestimmt werden:

n  = ƒ (Re) The mixing time depends, among other things, on the Reynolds number, which in turn depends on the shape of the reactor, the speed and the type of stirrer, but also on the density and viscosity of the liquid system. A scale transfer of the mixing time is difficult and always subject to errors, since there is no reliable literature data for the calculation of non-Newtonian liquids, such as dispersions. For simple model calculations, the literature (see, for example, Ullmanns Encyklopadie der techn. Chemie, 4th edition, volume 2, pages 259 ff, in particular pages 263-264 and Fig. 9) provides idealized relationships for liquids without differences in density or viscosity, one allow rough calculation of a minimum mixing time. Based on the viscosity η of the liquid medium, its density

The Reynolds number of the system can be calculated from the stirrer speed n and the stirrer diameter d taking into account the boundary conditions and the mixing time  can be determined from this:

n  = ƒ (Re)

Analogously to these equations, in the case of an anchor stirrer and an aqueous polymer dispersion with a viscosity of 30 mPas and a density of 1 g / cm ³ , taking into account the geometric similarities, the mixing time for idealized stirred reactors (laboratory, pilot plant, production scale), the results of which were calculated the table below shows. The selected stirring speeds n are based on practical experience. As the blade diameter increases, the stirring speed and thus the shear stress on the material to be mixed increases. In order to obtain a comparable, constant power input into the different sized reactors and to avoid an excessive increase in the peripheral speed of the stirrer, it is common practice to reduce the speed of the stirrer as the reactor size increases. The table below shows that the mixing time changes with the size of the reactor. reactor rotational speed
n per second Stirrer diameter (mm) circumferential speed ReZahl mixing time
(Sec.) laboratory 2.5 110 860 1000 40 pilot 1 1100 3400 40000 60 production 0666 2500 5200 140000 120

A simple transfer of the post-treatment relationships from the Laboratory scale, as described in the prior art, on Larger stirred tanks are therefore not possible.

If the height / diameter ratio (H / D) of the reactors changes to a greater degree of slenderness of the reactors, as is also preferred because of the favorable heat dissipation, the mixing time increases greatly. For a cross bar stirrer, Ullmann (loc.cit.) States: n = 16.5 x (H / D) ^2.6 . If a H / D value of 2 to 2.5 is typical for a production reactor, the mixing time increases by a factor of 5 to 10, although the crossbar stirrer is a better mixing stirrer than an anchor stirrer.

According to the inventive method, the liquid system in Production reactor have the shortest possible mixing time. To in addition to long-term dosing of at least one of the Components of the initiator system used for an effective To stir in the added substances. This can be done once through a suitable choice of the geometric parameters of the production reactor, the choice of a very effective stirrer with suitable Speeds or combinations thereof. So can a reduction in the mixing time by using a wall-mounted Spiral stirrer or coaxial stirrer. It is also for that also the use of a multi-stage wall-mounted stirrer strong axial mixing effect. It is under one wall-mounted stirrer understood one where the ratio Stirrer diameter to reactor diameter, reduced by the twice the width of any current breaker, at least reached the value 0.9. Each stirrer has a radial, i.e. a flow direction directed against the reactor wall, the axial flow direction, however, is little with many stirrers pronounced. Especially through the use of a multi-stage Stirrer, in which several along the axial, vertical stirrer axis Stirrers are attached, the axial mixing is increased. Especially with the usual feed of the reaction components from above through the reactor cover or from below the reactor base is subjected to the highest possible axial mixing needed to achieve a quick stir-in and a reaction to reduce the reaction component already during the mixing time. You can also use a wall-mounted MIG or INTERMIG stirrer, an impeller, propeller or disc stirrer or a wall-mounted anchor stirrer or wall-mounted Lattice mixer the mixing time can be reduced. It is often advantageous, the effect of the stirrer still in the production reactor built-in baffles, baffles or others To increase flow deflections.

In addition to the choice of stirrer type and other reactor internals set stirrer speed, viscosity, density, reagent type and concentration and the dosing time significantly influences the mixing time factors that can be optimized. The viscosity of the fluid Medium has an extremely high influence on the mixing time behavior of the reactor. For this reason, with heterophase polymers, i.e. Polymer dispersions and polymer suspensions, particularly low mixing times can be realized because of this Systems have significantly lower viscosities than solutions or melting.

If an added agent is to be added as a component of a Initiator system in an imaginary volume element of the reaction mass react with a residual monomer, the agent must be unused get there. If side reactions occur beforehand, see above this extends the mixing time required for rapid side reactions can become infinite. A dosage is necessary for thorough mixing during a period (dosing time) necessary in any case longer than the specific mixing time of the liquid system used Production reactor is. According to the method according to the invention the dosing time should be 10 to 250 times, especially that 20 to 100 times and most preferably 25 to 50 times the mixing time of the liquid system in the production reactor used be. With mixing times of 1 to 30 minutes the above information in about dosing times from 10 minutes to 20 hours, especially 20 minutes to 10 hours and preferred 30 minutes to 5 hours.

The dosing time also depends on the reaction rate of the radical-releasing system used, from its Side reactions and generally from its half-life at the the post-treatment reaction temperature. At a high reaction temperature depending on the half-life of the decaying initiator and set system parameters, e.g. pH, a Part of the initiator added during the mixing time disintegrated. The dosing time should be at least the sum of Half-life of the initiator system at the reaction temperature used plus 10 to 20 times the mixing time. at Two-component redox initiators should also be noted that in particular with simultaneous addition of the components at higher Concentration of component A at the component feed point B react component B without a monomer-reducing effect can. A spatially separate addition of the oxidation and the reduction component for effective monomer reduction significant in their reaction, if not already through an appropriate timing of the individual components a build-up of large concentration gradients avoided becomes. It is particularly disadvantageous to build up concentration gradients with regard to component A and component B of the initiator system at the same reactor end (top or bottom) what to a fast, but very ineffective in the residual monomer reduction Reaction of the components can lead to each other.

According to at least one of the initiator components the inventive method can be such that a Solution of the component to be added in a thin stream directly is placed on the surface of the liquid system in the reactor. However, it is more preferred that the component is subsequently added Initiator system for the reaction mixture from below through an opening in the reactor floor or through an opening in the reactor side wall make, especially when using a wall-mounted Stirrer. The metered solution thus appears in the highest zones Turbulence. Dosing via is particularly advantageous the inner opening of a hollow stirrer, the metered solution is permanently led into the zones of high turbulence. Against when fed through an opening in the reactor side wall and using an anchor stirrer only then entry into A turbulent zone is reached when the stirrer is just entering the inlet happened in the side wall. A is also preferred Dosing via a rotating one above the liquid system Foam breaker. This destroys it by its rotating movement mechanically by intensive stirring and mixing formed foam, which also contributes to the solution that the liquid surface irrigated.

The temperature of the reaction mixture in the production reactor during the metering of the last added component depends partly according to the redox initiator system used and its Half-life at the reaction temperature.

The reaction temperature is generally 20 to 140 ° C, especially at 30 to 120 ° C and with dispersions and suspensions preferably at 30 to 95 ° C.

The process is practical with all common redox initiator systems feasible, especially at not so high reaction temperatures are well effective and watery with post-treatment Polymer dispersions or suspensions, a sufficient one Have solubility in aqueous systems. The next to the oxidizer and the reducing agent frequently used metal ions are added to one of the components or lie there already distributed homogeneously in the reaction medium, e.g. as Component from the main polymerization. Examples of the redox reaction catalyzing metals are salts and complexes of iron, Copper, manganese, silver, platinum, vanadium, nickel, chrome, Palladium or cobalt, the metals each in different Oxidation stages can be present. Examples of suitable oxidation components of the redox systems are water-soluble hydroperoxides, tert-butyl hydroperoxide, cumene hydroperoxide, hydrogen peroxide or an ammonium or alkali salt of peroxydisulfuric acid.

Examples of suitable reducing agents are ascorbic acid, isoascorbic acid, organic compounds with thiol or disulfide groups, reducing inorganic alkali and ammonium salts of sulfur-containing acids, such as sodium sulfite, disulfite, thiosulfate, hydrosulfite, sulfide, hydrosulfide or dithionite, formamidine sulfinic acid, Hydroxymethanesulfinic acid, acetone bisulfite, Amines such as ethanolamine or endiols. Even under normal conditions gaseous oxidizing agents such as oxygen, ozone or air or gaseous reducing agents such as sulfur dioxide can be classified as Use reaction components of the redox system. The components of the redox system can be used simultaneously or one after the other Add reaction mixture (e.g. both from above, one from above, one from below), with a spatially separate addition (reactor cover, Reactor floor, reactor side wall) is very advantageous. It it is also possible that the reaction mixture is already out of phase the main polymerization, which generally leads to monomer conversion 90, in particular 95 and preferably 99% by weight is still sufficient oxidizing agent, reducing agent or contains metal catalyst, so that only in the aftertreatment the missing component is dosed in sufficient quantity must become. The quantities of the redox initiator components used it should be noted that usually amounts from 0.01 to 0.5, in particular 0.05 to 0.4 and preferably 0.2 to 0.3% by weight to the total mass of those used for the main polymerization Monomers are used.

Instead of redox systems, initiators can also act as radical sources such as dialkyl or diacyl peroxides, azo compounds, perketals or Peracetale can be used, or by high energy Radiation or radicals generated by ultrasound.

The process according to the invention is particularly suitable for reducing the residual monomers in liquid systems of acrylate, methacrylate (esters of acrylic acid or methacrylic acid with C ₁ -C ₁₂ alkanols, in particular C ₁ -C ₈ alkanols, methyl, ethyl , n-butyl and 2-ethylhexyl acrylate and methacrylate are particularly preferred), styrene, vinyl chloride or vinyl acetate copolymers, such as styrene-butadiene copolymers or ethylene-vinyl acetate copolymers. In addition to the main monomers of the type mentioned, the monomer mixtures used for the polymerization can also be used in smaller amounts, in particular 0.01 to 10% by weight of the total amount of monomers, polar monomers such as acrylic acid, methacrylic acid, methacrylamide, acrylamide and / or their N-methylol derivatives, Contain maleic acid or its anhydride or hydroxyalkyl (meth) acrylates. The process is particularly suitable for reducing residual monomers in aqueous dispersions.

The preparation of those used according to the invention for an aftertreatment Polymers are known to the person skilled in the art from the specialist literature.

The following examples are intended to illustrate the invention, however not restrict. Unless otherwise stated, refer to Parts and percentages by weight. The given in the examples Residual amounts of monomer were determined by gas chromatography (GC). The solids contents (FG) became gravimetric after drying certainly. The LD value is the translucency, one 0.01% by weight sample of the corresponding polymer dispersion Layer thickness of 25 mm compared to pure water. The particle weight distribution (TGV) was created by capillary hydrodynamic Fractionation or determined using an ultracentrifuge.

The viscosity of the dispersions (mPas) was determined using a commercially available Rotational viscometer (Rheomat) at a shear rate of 500 / second determined.

example 1

1a. In a 1.3 m ³ production reactor with an anchor stirrer (ratio of stirrer blade to reactor diameter d / D: 0.7; stirrer speed: 50 revolutions per minute), the metal parts of which were electropolished, 317 kg of water and 4.4 kg of emulsifier ( EM) submitted and mixed with 2% of the monomer emulsion ME1 below. Monomer emulsion ME1: 65 kg water 14.6 kg a 28% aqueous emulsifier solution of a neutralized C ₁₂ -C ₁₄ fatty alcohol ether sulfate with 3 moles of ethylene oxide 0.5 kg Caustic soda (50%) 54.7 kg Methacrylamide (15%) 54.7 kg N-methylol methacrylamide (15%) 12.3 kg methacrylic acid 65.6 kg methyl methacrylate 373.1 kg n-butyl acrylate The mixture was then heated to 80 ° C. and 10% of an initiator solution consisting of 1.23 kg sodium persulfate in 48 kg water was added. After 5 minutes, the remainder of the monomer emulsion was added over 2 hours, the rest of the initiator solution over 2.5 hours, and then the polymerization batch was kept in the reactor at 80 ° C. for another hour. A sample A was then removed and its residual monomer content was determined by gas chromatography (sample 1A). The viscosity of the dispersion was approximately 30 mPas. The mixing time of the reactor was 1.2 minutes.

1b. After treatment: An inlet IA consisting of 11.5 kg of a 9% aqueous solution of tert-butyl hydroperoxide, as well as a Inlet IB, consisting of a mixture of 18.6 kg Water, 0.58 kg sodium disulfite and 0.33 kg acetone, were simultaneously during a dosing time of 3 minutes on opposite sides through the reactor cover metered into the reaction mixture. It was the batch cooled to 30 ° C. in 90 minutes and then 20, 40, 60, and 90 minutes one product sample each for gas-chromatic determination of the residual monomer content taken (samples 1B, 1C, 1D, 1E). After the 90th The batch was run for 25 minutes with 25% aqueous ammonia neutralized and filtered through a 125 µm filter. It became a coagulate-free polymer dispersion with a solids content of 40.2%, an LD value of 92% and a pH of 7.2 obtained. The particle weight distribution is monomodal with a narrow distribution. Approximately 1 hour after finishing the post-treated Polymer dispersion again became a sample (sample 1F) taken for the determination of the residual monomer content. The determined residual monomer contents of the samples 1A-1F shows Table 1.

Example 2

2a. The procedure was exactly as given in Example 1a and taken a sample 2A.

2 B. After treatment: The procedure was as described in Example 1b, but was carried out an inlet IIA, which corresponded to inlet IA, during a Duration of 5 minutes through the reactor lid of the reaction mixture metered in and stirred into this for 5 minutes. An inlet IIB (reducing agent), the inlet IB corresponded in its composition, was during a dosing time 90 minutes during the 90 minute cooling period continuously metered into the reaction mixture. As in Example 1b were given, samples for the determination of the residual monomer content taken (samples 2B, 2C, 2D, 2E, 2F). The results of the investigation shows Table 1.

Example 3

3a. The procedure is as given in Example 1a and a Sample taken for residual monomer determination (sample 3A).

3b. After treatment: The procedure is as given in Example 1b, but a Inlet IIIA, which corresponded to inlet IA, for 5 minutes metered through the reactor lid of the reaction mixture, and then stirred in for 5 minutes. An inflow IIIB that the inflow corresponded to IB, was during the 1.5 hour cooling dosed from 80 ° C to 30 ° C through the reactor floor (dosing time 90 minutes). As indicated in Example 1b Samples taken to determine the residual monomer content (samples 3B, 3C, 3D, 3E, 3F). The results are shown in Table 1.

Example 4

4a. The procedure was as given in Example 1a and a sample was taken to determine the amount of residual monomers (Sample 4A).

4b. After treatment: The procedure was as given in Example 1b, but an oxidant feed IVA, which in its composition corresponded to the inlet IA, as well as a reducing agent inlet IVB, whose composition corresponded to the addition IB, at the same time during the 1.5 hour cooling period steadily on opposite sides through the reactor cover metered into the reaction mixture from above (metering time of both Inlets 90 minutes each). As indicated in Example 1b Samples taken to determine the amount of residual monomers (samples 4B, 4C, 4D, 4E, 4E). The results are shown in Table 1.

Example 5

5a. The procedure was as given in Example 1a, but was the monomer emulsion ME for 4 hours and the initiator solution metered in over 4.5 hours and then the batch kept at the polymerization temperature for a further 30 minutes. As indicated in Example 1a, a sample was used for the determination of the residual monomer content after the main polymerization (Sample 5A).

5b. After treatment: The procedure was as in Example 1b, but the feeds were of the redox initiator components VA and VB, which are the inlets IA and IB correspond, at the same time, but spatially separated on opposite sides from above through the reactor cover during a dosing time of 60 minutes 80 ° C heated reaction mixture metered. After 20, 40 and 60 Minutes of dosing were samples from the reaction mixture taken to determine the residual monomer content (samples 5B, 5C, 5D). The results are shown in Table 1.

Example 6

6a. The procedure was as given in Example 1a, and one Sample (sample 6A) taken for residual monomer determination.

6b. Aftertreatment: The procedure was as described in Example 1b. In the feeds of the redox initiator components, feed VIA, which corresponds to feed IA, was metered into the reaction mixture at 80 ° C. from above through the reactor lid over 15 minutes. The reaction mixture was then cooled to 30 ° C. Feed VIB, which corresponds in its composition to the reducing agent from feed IB, was metered in from above through the reactor lid of the reaction mixture during a metering time of 120 minutes. After 20, 40, 60, 90 and 120 minutes, samples were taken to determine the residual monomer content (samples 6B, 6C, 6D, 6E, 6F). The results are shown in Table 1. Type and time of addition of tert-butyl hydroperoxide solution (Ox) and acetone bisulfite solution as reducing agent (Red) in the aftertreatment as well as residual monomer contents of n-butyl acrylate (BA) and methyl methacrylate (MMA) of the product samples taken before, during and after the aftertreatment Ex.1 Ex.2 EX3 EX4 Bsp.5 Bsp.6 Ox addition: place above above above above above above Time (min.) 3 5 5 90 60 15 Encore speech place above above below above above above Time (min.) 3 90 90 90 60 120 encore simultaneously + - - + + - successively - + + - - + Samples A Baseline (1A) (2A) (3A) (4A) (5A) (6A) BA (ppm) 2800 2800 3000 2600 2800 3000 MMA (ppm) 20 10 20 10 20 10 Samples B after 20 min. (1B) (2 B) (3B) (4B) (5B) (6B) BA (ppm) 1500 1600 850 2100 1500 1300 MMA (ppm) 15 <10 <10 15 <10 <10 Samples C after 40 min. (1C) (2C) (3C) (4C) (5C) (6C) BA (ppm) 1300 1000 580 2100 540 580 MMA (ppm) 10 <10 <10 10 <10 <10 Samples D after 60 min. (1D) (2D) (3D) (4D) (5D) (6D) BA (ppm) 1100 830 180 1300 170 320 MMA (ppm) 10 <10 <10 10 <10 <10 Samples E after 90 min. (1E) (2E) (3E) (4E) (5E) (6E) BA (ppm) 1100 480 100 370 - 150 MMA (ppm) <10 <10 <10 <10 - <10 Samples F after ≥120 min. (1F) (2F) (3F) (4F) (5F) (6F) BA (ppm) 1050 480 100 300 - 140 MMA (ppm) <10 <10 <10 <10 - <10

Examples 7 to 12 and comparative experiment VV

Aftertreatment of highly concentrated polymer dispersions with different Redox systems and types of addition:

7-12a. Production of higher concentrated polymer dispersions

A 7-12V monomer emulsion of the following composition was prepared: 90 kg water 18 kg Sodium salt of dodecylbenzenesulfonic acid as a 15% aqueous solution 0.082 kg Iron (II) sulfate heptahydrate 0.70 kg tert-dodecyl mercaptan 38 kg N-methylol methacrylamide (15%) 29 kg acrylonitrile 6 kg acrylic acid 539 kg n-butyl acrylate

In a vessel equipped with an anchor stirrer with a volume of 1.6 m ³ , which is 2/3 full and has a liquid height to reactor diameter ratio of h / D = 0.88, 105 kg of water were placed, with stirring 40 rpm heated and mixed with 0.5% of the monomer emulsion 7-12V. After stirring for one minute, 20% of a solution of 2.9 kg sodium persulfate in 110 kg water was added. After 10 minutes, the rest of the monomer emulsion 7-12V and the rest of the initiator solution were continuously added over 4 hours. After the addition was complete, a sample (Sample A) was taken.

7-12b. Aftertreatment of the polymer dispersions after Examples 7-12 and comparison test (VV)

Beispiel 7:

18 %ige wässrige Lösung von Ascorbinsäure

Beispiel 8:

18 %ige wässrige Lösung von Ascorbinsäure

Beispiel 9:

18 %ige wässrige Lösung von Rongalit® C.

Beispiel 10:

Mischung 10%ige Natriumdisulfitlösung mit Aceton (Gewichtsverhältnis 15:1) (ABS)

Beispiel 11:

16 %ige wässrige Mercaptoethanollösung (HO-CH₂-CH₂-SH) (ME)

Beispiel 12:

1,2 %ige Formamidinsulfinsäurelösung (FAS)

The polymer dispersions prepared were each mixed with 11.5 kg of a 10% strength solution of tert-butyl hydroperoxide. Then a solution of the following reducing agents was metered in over 2 hours:
Comparative experiment VV: water without reducing agent

Example 7:

18% aqueous solution of ascorbic acid

Example 8:

18% aqueous solution of ascorbic acid

Example 9:

18% aqueous solution of Rongalit® C.

Example 10:

Mixture of 10% sodium disulfite solution with acetone (weight ratio 15: 1) (ABS)

Example 11:

16% aqueous mercaptoethanol solution (HO-CH ₂ -CH ₂ -SH) (ME)

Example 12:

1.2% formamidine sulfinic acid solution (FAS)

During the addition, a sample was taken after 30, 60 and 120 minutes, in which the content of the residual monomers n-butyl acrylate (BA) and acrylonitrile (AN) was determined by gas chromatography. After cooling, the aftertreated polymer dispersion was adjusted to a pH of about 7.5 with a 25% aqueous ammonia solution. The examples and their results are summarized in Tables 2 and 3. VV Bsp.7 Bsp.8 Bsp.9 Bsp.10 Bsp.11 Bsp.12 Red.mittel: without + - - - - - - Ascorbic. - + + - - - - Rongalit C - - - + - - - SECTION - - - - + - - ME - - - - - + - FAS - - - - - - + Red-addition place above below above below above below above Duration (hrs.) 2 2 2 2 2 2 3 2 Parts per 100 Parts of monomer - 0.20 0.20 0.2 0.26 0.3 0.25 Solids content (%) 59.7 60.2 59.1 59.9 59.6 59.1 51.9 PH value 7.3 7.2 7.7 7.5 7.7 7.3 7.8 LT value 25% 26% 28% 26% 29% 23% 23% Visk.mPas - 105 42 42 72 84 15 Coagulum kg 0.01 0.01 0.25 0.50 0.02 0.05 0.50 Contents of residual monomers n-butyl acrylate (BA) and acrylonitrile (AN) in ppm of the product samples taken before, during and after the aftertreatment VV Bsp.7 Bsp.8 Bsp.9 Bsp.10 Bsp.11 Bsp.12 Samples A begin BA (ppm) 42000 55000 43000 43000 35000 44000 49000 AN (ppm) 2050 2400 2000 21000 1800 1800 2800 Samples B (30 min.) BA (ppm) 40000 15000 13000 13000 7200 18000 15000 AN (ppm) 2000 690 600 470 270 810 880 Samples C (60 min.) BA (ppm) 39000 10000 5700 7800 2400 6400 5600 AN (ppm) 1950 430 230 340 20 220 230 Samples D (120 min.) BA (ppm) 39000 610 660 1300 230 1000 580 AN (ppm) 1900 10 10 30 <10 <10 <10 Samples E end values BA (ppm) 38000 600 610 1200 230 850 290 AN (ppm) 1800 <10 <10 20 <10 <10 <10

Claims (8)

1. A process for reducing the residual monomer content in a liquid solution, mixture, melt, suspension or dispersion of a polymer prepared by free-radical polymerization (liquid system) by postpolymerization with addition of a redox initiator system at a reaction temperature appropriate to it, which comprises gradually metering at least one of the redox initiator components required to initiate polymerization of the residual monomers into the liquid system in a production reactor with a capacity of more than 20 liters and a defined and extremely short mixing time over a period (metering time) which is from about 10 to 250 times the mixing time of the liquid system in said reactor.
2. A process as claimed in claim 1, wherein measures to reduce the mixing time of the liquid system in the production reactor are taken before or during the metered addition of at least one of the initiator components.
3. A process as claimed in claim 1 or 2, wherein the addition of oxidation and reduction component of the initiator system to the liquid system takes place at spatially separate locations and simultaneously or in succession.
4. A process as claimed in claim 3, wherein one of the components of the initiator system is added to the production reactor from above and the other through the floor of said reactor.
5. A process as claimed in any of claims 1 to 4, wherein the mixing time of the liquid system in the production reactor is reduced by using a multistage close-clearance stirrer with a strong axial mixing effect in the reactor.
6. A process as claimed in any of claims 1 to 5, wherein the mixing time of the liquid system is reduced by the production reactor having one or more stirrers and also guide vanes, flow disruptors or flow diverters.
7. A process as claimed in any of claims 1 to 6, wherein the metering time is from 20 to 100 times the mixing time of the liquid system in the production reactor.
8. A process as claimed in any of claims 1 to 7, wherein the residual monomers whose content is to be reduced are esters of acrylic acid, esters of methacrylic acid, acrylonitrile and/or methacrylonitrile.